MARF Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track March 19, 2012
Expires: September 20, 2012
Extensions to DKIM for Failure Reportingdraft-ietf-marf-dkim-reporting-16
Abstract
This document presents extensions to the DomainKeys Identified Mail
(DKIM) specification to allow for detailed reporting of message
authentication failures in an on-demand fashion.
Status of this Memo
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Internet-Draft DKIM Reporting Extensions March 20121. Introduction
DomainKeys Identified Mail [DKIM] introduced a mechanism for message
signing and authentication. It uses digital signing to associate a
domain name with a message in a reliable manner. The verified domain
name can then be evaluated (e.g., checking advertised sender policy,
comparison to a known-good list, submission to a reputation service,
etc.).
Deployers of message authentication technologies are increasingly
seeking visibility into DKIM verification failures and conformance
failures involving the published signing practices (e.g., Author
Domain Signing Practices, [ADSP]) of an Administrative Management
Domain (ADMD; see [EMAIL-ARCH]).
This document extends [DKIM] and [ADSP] to add an optional reporting
address and some reporting parameters. Reports are generated using
the format defined in [I-D.IETF-MARF-AUTHFAILURE-REPORT].
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Internet-Draft DKIM Reporting Extensions March 20122. Definitions2.1. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
2.2. Notation
Certain properties of email messages described in this document are
referenced using notation found in [EMAIL-ARCH] (e.g.,
"RFC5322.From").
2.3. Imported Definitions
Numerous DKIM-specific terms used here are defined in [DKIM]. The
definitions of the [ABNF] tokens "domain-name" and "dkim-quoted-
printable" can also be found there.
2.4. Other Definitions
report generator: A report generator is an entity that generates and
sends reports. For the scope of this document, the term refers to
Verifiers, as defined in Section 2.2 of [DKIM], with the added
capability to generate authentication failure reports according to
this specification.
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Internet-Draft DKIM Reporting Extensions March 20123. Optional Reporting for DKIM
A domain name owner employing [DKIM] for email signing and
authentication might want to know when signatures that ought to be
verifiable are not successfully verifying. Currently there is no
such mechanism defined.
This section adds optional "tags" (as defined in [DKIM]) to the DKIM-
Signature header field and the DKIM key record in the DNS, using the
formats defined in that specification.
3.1. Extension DKIM Signature Tag
The following tag is added to DKIM-Signature header fields when a
Signer wishes to request that reports of failed verifications be
generated by a Verifier:
r= Reporting Requested (plain-text; OPTIONAL; no default). If
present, this tag indicates that the Signer requests that
Verifiers generate a report when verification of the DKIM
signature fails. At present, the only legal value is the single
character "y". A complete description and illustration of how
this is applied can be found in Section 3.3.
ABNF:
sig-r-tag = %x72 *WSP "=" *WSP %x79
; "r=y" (lower-case only)
3.2. DKIM Reporting TXT Record
When a Signer wishes to advertise that it wants to receive failed
verification reports, it places in the DNS a TXT resource record
(RR). The RR contains a sequence of tag-value objects in a format
similar to DKIM key records (see Section 3.6.1 of [DKIM]), but it is
entirely independent of those key records and is found at a different
name. The tag-value objects in this case comprise the parameters to
be used when generating the reports. A report generator will request
the content of this record when it sees an "r=" tag in a DKIM-
Signature header field.
Section 3.6.2.2 of [DKIM] provides guidance with respect to handling
of a TXT RR that comprises multiple distinct strings ("character-
strings" in the parlance of [DNS]). The same process MUST be applied
here.
Implementations MUST support all tags defined in this document, and
any other tag found in the content of the record that is not
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recognized by an implementation MUST be ignored. See Section 7.3 for
details about finding or registering extension tags.
The initial list of tags supported for the reporting TXT record is as
follows:
ra= Reporting Address (plain-text; OPTIONAL). A dkim-quoted-
printable string (see Section 2.11 of [DKIM]) containing the
local-part of an email address to which a report SHOULD be sent
when mail fails DKIM verification for one of the reasons
enumerated below. The value MUST be interpreted as a local-part
only. To construct the actual address to which the report is
sent, the Verifier simply appends to this value an "@" followed by
the domain name found in the "d=" tag of the DKIM-Signature header
field. Therefore, an ADMD making use of this specification MUST
ensure that an email address thus constructed can receive reports
generated as described in Section 6. ABNF:
rep-ra-tag = %x72.61 *WSP "=" *WSP dkim-quoted-printable
; "ra=..." (lower-case only for the tag name)
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is an integer from 0 to 100 inclusive that
indicates what percentage of incidents of signature authentication
failures, selected at random, are to cause reports to be
generated. The report generator SHOULD NOT issue reports for more
than the requested percentage of incidents. Report generators MAY
make use of the "Incidents:" field in [ARF] to indicate that there
are more reportable incidents than there are reports. ABNF:
rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
; "rp=..." (lower-case only)
rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing those
conditions under which a report is desired. See Section 5.1 for a
list of valid tokens. ABNF:
rep-rr-type = ( "all" / "d" / "o" / "p"/ "s" / "u" / "v" / "x" )
rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type
*WSP *( ":" *WSP rep-rr-type )
; "rr=..." (lower-case only for the tag name)
rs= Requested SMTP Error String (text; OPTIONAL; no default). The
value is a dkim-quoted-printable string that the publishing ADMD
requests be included in [SMTP] error strings if messages are
rejected during the delivery SMTP session. ABNF:
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Internet-Draft DKIM Reporting Extensions March 2012
rep-rs-tag = %x72.73 *WSP "=" dkim-quoted-printable
; "rs=..." (lower-case only for the tag name)
In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be
ignored, and the report generator MUST NOT issue a report.
3.3. DKIM Reporting Algorithm
Report generators MUST apply the following algorithm, or one
semantically equivalent to it, for each DKIM-Signature header field
whose verification fails for some reason. Note that this processing
is done as a reporting extension only; the outcome of the specified
DKIM evaluation MUST be otherwise unaffected.
1. If the DKIM-Signature field did not contain a valid "r=" tag,
terminate.
2. Issue a [DNS] TXT query to the name that results from appending
the value of the "d=" tag in the DKIM-Signature field to the
string "_report._domainkey.". For example, if the DKIM-
Signature header field contains "d=example.com", issue a DNS TXT
query to "_report._domainkey.example.com".
3. If the DNS query returns anything other than RCODE 0 (NOERROR),
or if multiple TXT records are returned, terminate.
4. If the resultant TXT is in several string fragments, concatenate
them as described in Section 3.6.2.2 of [DKIM].
5. If the TXT content is syntactically invalid (see Section 3.2),
terminate.
6. If the reason for the signature evaluation failure does not
match one of the report requests found in the "rr=" tag (or its
default value), terminate.
7. If a report percentage ("rp=") tag was present, select a random
number between 0 and 99, inclusive; if the selected number is
not lower than the tag's value, terminate.
8. If no "ra=" tag was present, skip this step and the next one.
Otherwise, determine the reporting address by extracting the
value of the "ra=" tag and appending to it "@" followed by the
domain name found in the "d=" tag of the DKIM-Signature header
field.
9. Construct and send a report in compliance with Section 6 of this
document that includes as its intended recipient the address
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Internet-Draft DKIM Reporting Extensions March 2012
constructed in the previous step.
10. If the [SMTP] session during which the DKIM signautre was
evaluated is still active and the SMTP server has not already
given its response to the DATA command that relayed the message,
and an "rs=" tag was present in the TXT record, the SMTP server
SHOULD include the decoded string found in the "rs=" tag in its
SMTP reply to the DATA command.
In order to thwart attacks that seek to convert report generators
into unwitting denial-of-service attack participants, a report
generator SHOULD NOT issue more than one report to any given domain
as a result of a single message. Further, a report generator SHOULD
establish an upper bound on the number of reports a single message
can generate overall. For example, a message with three invalid
signatures, two from example.com and one from example.net, would
generate at most one report to each of those domains.
This algorithm has the following advantages over previous pre-
standardization implementations, such as early versions of
[OPENDKIM]:
a. If the DKIM signature fails to verify, no additional DNS check is
made to see if reporting is requested; the request is active in
that it is included in the DKIM-Signature header field.
(Previous implementations included the reporting address in the
DKIM key record, which is not queried for certain failure cases.
This meant, for full reporting, that the key record had to be
retrieved even when it was not otherwise necessary.)
b. The request is confirmed by the presence of a corresponding TXT
record in the DNS, since the Signer thus provides the parameters
required to construct and send the report. This means a
malicious Signer cannot falsely assert that someone else wants
failure reports and cause unwanted mail to be generated. It can
cause additional DNS traffic against the domain listed in the
"d=" signature tag, but negative caching of the requested DNS
record will help to mitigate this issue.
c. It is not possible for a Signer to direct reports to an email
address outside of its own domain, preventing distributed email-
based denial-of-service attacks.
See Section 8.4 for some considerations regarding limitations of this
mechanism.
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Internet-Draft DKIM Reporting Extensions March 20124. Optional Reporting Address for DKIM-ADSP
A domain name owner employing Author Domain Signing Practices [ADSP]
may also want to know when messages are received without valid author
domain signatures. Currently there is no such mechanism defined.
This section adds the following optional "tags" (as defined in
[ADSP]) to the DKIM ADSP records, using the form defined in that
specification:
ra= Reporting Address (plain-text; OPTIONAL; no default). The value
MUST be a dkim-quoted-printable string containing the local-part
of an email address to which a report SHOULD be sent when mail
claiming to be from this domain failed the verification algorithm
described in [ADSP], in particular because a message arrived
without a signature that validates, which contradicts what the
ADSP record claims. The value MUST be interpreted as a local-part
only. To construct the actual address to which the report is
sent, the Verifier simply appends to this value an "@" followed by
the domain whose policy was queried in order to evaluate the
sender's ADSP, i.e., the RFC5322.From domain of the message under
evaluation. Therefore, a signer making use of this extension tag
MUST ensure that an email address thus constructed can receive
reports generated as described in Section 6. ABNF:
adsp-ra-tag = %x72.61 *WSP "=" dkim-quoted-printable
; "ra=..." (lower-case only for the tag name)
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is a single integer from 0 to 100 inclusive
that indicates what percentage of incidents of ADSP evaluation
failures, selected at random, should cause reports to be
generated. The report generator SHOULD NOT issue reports for more
than the requested percentage of incidents. An exception to this
might be some out-of-band arrangement between two parties to
override it with some mutually agreed value. Report generators
MAY make use of the "Incidents:" field in [ARF] to indicate that
there are more reportable incidents than there are reports. ABNF:
adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
; "rp=..." (lower-case only)
rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing those
conditions under which a report is desired. See Section 5.2 for a
list of valid tokens. ABNF:
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Internet-Draft DKIM Reporting Extensions March 20125. Requested Reports
The "rr" tags defined above allow a Signer to specify the types of
errors about which it is interested in receiving reports. This
section defines the error types and corresponding token values.
Verifiers MUST NOT generate reports for incidents that do not match a
requested report, and MUST ignore requests for reports not included
in this list.
5.1. Requested Reports for DKIM Failures
The following report requests are defined for DKIM keys:
all All reports are requested.
d Reports are requested for signature evaluation errors that
resulted from DNS issues (e.g., key retrieval problems).
o Reports are requested for any reason related to DKIM signature
evaluation not covered by other report requests listed here.
p Reports are requested for signatures that are rejected for local
policy reasons at the Verifier that are related to DKIM signature
evaluation.
s Reports are requested for signature or key syntax errors.
u Reports are requested for signatures that include unknown tags in
the signature field.
v Reports are requested for signature verification failures or body
hash mismatches.
x Reports are requested for signatures rejected by the Verifier
because the expiration time has passed.
5.2. Requested Reports for DKIM ADSP Failures
The following report requests are defined for ADSP records:
all All reports are requested.
o Reports are requested for any [ADSP]-related failure reason not
covered by other report requests listed here.
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p Reports are requested for messages that are rejected for local
policy reasons at the Verifier that are related to [ADSP].
s Reports are requested for messages that have a valid [DKIM]
signature but do not match the published [ADSP] policy.
u Reports are requested for messages that have no valid [DKIM]
signature and do not match the published [ADSP] policy.
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Internet-Draft DKIM Reporting Extensions March 20126. Report Generation
This section describes the process for generating and sending reports
in accordance with the request of the signer and/or sender as
described above.
6.1. Report Format
All reports generated as a result of requests contained in these
extension parameters MUST be generated in compliance with [ARF] and
its extension specific to this work,
[I-D.IETF-MARF-AUTHFAILURE-REPORT]. Moreover, because abuse reports
from unverified sources might be handled with some skepticism, report
generators are strongly advised to use [DKIM] to sign reports they
generate.
6.2. Other Guidance
Additional guidance about the generation of these reports can be
found in [I-D.IETF-MARF-AS], especially Section 9.
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Internet-Draft DKIM Reporting Extensions March 20128. Security Considerations
Security issues with respect to these reports are similar to those
found in [DSN].
8.1. Inherited Considerations
Implementers are advised to consider the Security Considerations
sections of [DKIM], [ADSP], [I-D.IETF-MARF-AS], and
[I-D.IETF-MARF-AUTHFAILURE-REPORT]. Many security issues related to
this draft are already covered in those documents.
8.2. Report Volume
It is impossible to predict the volume of reports this facility will
generate when enabled by a report receiver. An implementer ought to
anticipate substantial volume, since the amount of abuse occurring at
receivers cannot be known ahead of time, and may vary rapidly and
unpredictably.
8.3. Deliberate Misuse
Some threats caused by deliberate misuse of this error reporting
mechanism are discussed in Section 3.3, but they warrant further
discussion here.
The presence of the DNS record that indicates willingness to accept
reports opens the recipient to abuse. In particular, it is possible
for an attacker to attempt to cause a flood of reports toward the
domain identified in a signature's "d=" tag in one of these ways:
1. Alter existing DKIM-Signature header fields by adding an "r=y"
tag (and possibly altering the "d=" tag to point at the target
domain);
2. Add a new but bogus signature bearing an "r=y" tag and a "d=" tag
pointing at the target domain;
3. Generate a completely new message bearing an "r=y" tag and a "d="
tag pointing at the target domain.
Consider, for example, the situation where an an attacker sends out a
multi-million-message spam run, and includes in the messages a fake
DKIM signature containing "d=example.com; r=y". It won't matter that
those signatures couldn't possibly be real: each will fail
verification, and any implementations that support this specification
will report those failures, in the millions and in short order, to
example.com.
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Internet-Draft DKIM Reporting Extensions March 2012
Implementers are therefore strongly advised not to advertise the DNS
record specified in this document except when failure reports are
desired. Upon doing so, unexpected traffic volumes and attacks
should be anticipated.
Negative caching offers some protection against this pattern of
abuse, although it will work only as long as the negative time-to-
live on the relevant SOA record in the DNS.
Positive caching of this DNS reply also means turning off the flow of
reports by removing the record is not likely to have immediate
effect. A low time-to-live on the record needs to be considered.
8.4. Unreported Fraud
An attacker can craft fraudulent DKIM-Signature fields on messages,
without using "r=" tags, and avoid having these reported. The
procedure described in Section 3.3 does not permit the detection and
reporting of such cases.
It might be useful to some Signers to receive such reports, but the
mechanism does not support it. To offer such support, a Verifier
would have to violate the first step in the procedure and continue
even in the absence of an "r=" tag. Although that would enable the
desired report, it would also create a possible denial-of-service
attack: such Verifiers would always look for the reporting TXT
record, so a generator of fraudulent messages could simply send a
large volume of messages without an "r=" tag to a number of
destinations. To avoid that outcome, reports of fraudulent DKIM-
Signature header fields are not possible using the published
mechanism.
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Internet-Draft DKIM Reporting Extensions March 2012Appendix B. Examples
This section contains examples of the use of each of the extensions
defined by this document.
B.1. Example Use of DKIM Signature Extension Tag
A DKIM-Signature field including use of the extension tag defined by
this document:
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
d=example.com; s=jan2012; r=y;
h=from:to:subject:date:message-id;
bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8
Example 1: DKIM-Signature field using this extension
This example DKIM-Signature field contains the "r=" tag that
indicates reports are requested on verification failure.
Assuming the public key retrieved from the DNS and processed
according to [DKIM] would determine that the signature is invalid, a
TXT query will be sent to "_report._domainkey.example.com" to
retrieve a reporting address and other report parameters as described
in Section 3.3.
B.2. Example DKIM Reporting TXT Record
An example DKIM Reporting TXT Record as defined by this document:
ra=dkim-errors; rp=100; rr=v:x
Example 2: Example DKIM Reporting TXT Record
This example, continuing from the previous one, shows a message that
might be found at "_report._domainkey.example.com" in a TXT record.
It makes the following requests:
o Reports about signature evaluation failures should be send to the
address "dkim-errors" at the signer's domain;
o All (100%) incidents should be reported;
o Only reports about signature verification failures and expired
signatures should be generated.
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Internet-Draft DKIM Reporting Extensions March 2012B.3. Example Use of DKIM ADSP Extension Tags
A DKIM ADSP record including use of the extensions defined by this
document:
dkim=all; ra=dkim-adsp-errors; rr=u
Example 3: DKIM ADSP record using these extensions
This example ADSP record makes the following assertions:
o The sending domain (i.e. the one that is advertising this policy)
signs all mail it sends;
o Reports about ADSP evaluation failures should be send to the
address "dkim-adsp-errors" at the Author's domain;
o Only reports about unsigned messages should be generated.
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